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0 °C

To as stirred solution of Boc-L-Nle-ol (0.50 g, 1.9 mmol) in dry tetrahydrofuran (15 mL), cooled at 0° C. under nitrogen, NaH (0.05 g, 2.1 mmol) was added. The reaction mixture was stirred for 45 min at 0° C., followed by addition of 18-crown-6 (0.25 g, 0.9 mmol) and subsequently a solution of ethyl bromoacetate (0.47 g, 2.8 mmol) in dry tetrahydrofuran (4 mL) was added dropwise at 0° C. The reaction mixture was stirred for 1 h at 0° C. and overnight at room temperature. The organic solvent was evaporated under reduced pressure and the residue was purified by column-chromatography using chloroform as eluent. Yield 55%; Oil; [a]D=−13.9 (c 1CHCl3); 1H NMR (200 MHz, CDCl3) δ 4.85-4.78 (m, 1H), 4.17 (q, J=7.4 Hz, 2H), 4.03 (s, 2H), 3.72-3.58 (m, 1H), 3.56-3.41 (m, 2H), 1.62-1.21 (m, 18H), 0.85 (t, J=6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 170.3, 155.6, 78.9, 73.3, 68.4, 60.7, 50.3, 31.5, 28.3, 28.1, 22.5, 14.1, 13.9. Anal. Calcd for C15H29NO5: C, 59.38; H, 9.63; N, 4.62. Found: C, 59.16; H, 9.82; N, 4.51.

1. stirringThe reaction mixture was stirred for 1 h at 0° C. and overnight at room temperature
2. customThe organic solvent was evaporated under reduced pressure
3. customthe residue was purified by column-chromatography